This invention relates generally to a video analog-to-digital conversion (hereinafter referred to as the video "A/D converter" or alternately as the "A/D conversion circuit) circuit, and more particularly to a circuit for generating a clock signal which is synchronized to a digital video signal produced by the A/D converter.
In conventional video A/D converters designed for handling a video signal, a synchronizing signal is extracted from an input analog video signal by a synchronizing signal separator circuit. A clock signal is generated by a phase locked loop on the basis of the separated synchronizing signal. In other words, the video A/D converter contains, in addition to a conventional A/D converter, a voltage control oscillator (VCO) for generating the clock signal, a counter for frequency-dividing the clock signal to generate a frequency divided output signal having a frequency equal to that of the synchronizing signal and a phase comparator for comparing the phase of the frequency division output signal from the counter to the output of the separated synchronizing signal to provide a control voltage for the VCO. The clock signal from the VCO is supplied to the A/D converter which provides a digital (PCM) video signal output. The video A/D converter also includes a lowpass filter for preventing folded noise, the filter being located as a pre-stage circuit of the A/D converter.
The synchronizing signal in the synchronizing signal separator is obtained by a clipper (comparator) after frequency band limitation is made by a lowpass filter in order to remove any erroneous components, i.e., a chrominance component and a noise component contained in the video signal. Consequently and due to the lowpass filter, a drift is generated and since the lowpass filter is disposed before the A/D converter, a drift or fixed delay also occurs. Further, the A/D converter itself produces variations in conversion time. Therefore, synchronisation between the synchronizing signal in the digital video signal and the clock signal supplied to the A/D converter for effecting quantitization is inaccurate. The phase difference therebetween sometimes may reach as much as a full clock period.
If the abovementioned clock signal is then used to produce a control signal for mixing, for example, plural digital video signals in a digital video system where all video signals are processed in digital form, the phase difference between the synchronizing signal contained in the digital video signal and the clock signal causes a delicate time lag in a mixed video signal. In the worst case, horizontal blanking appears in the active video portion when a plurality of digital video processing equipment are connected in series in the digital video system.